There has hitherto been known a machine tool which carries out processing such as cutting by moving a tool relative to a workpiece. The machine tool comprises a spindle device including a spindle to which the tool is fitted and a motor or the like for rotating the spindle. The spindle device is generally formed so that the tool can be replaced, and a plurality of processes can be carried out by tool replacement.
Patent Literature 1 discloses a peripheral velocity switching device which can change a preload of a bearing of the spindle, and switches a bearing set of the spindle according to a rotational speed of the spindle. As disclosed, this switching device applies a preload to a large bearing set disposed in front of the spindle during low-speed rotation. When a switch piston is moved by hydraulic pressure, the preload is applied to the large bearing set. On the other hand, during high-speed rotation, the large bearing set is in a state of no preload application. As disclosed, a tapered part is formed at an end of the switch piston, and a preload is absorbed when the preload of a pressure fluid is excessively large.
Patent Literature 2 discloses a spindle device which includes a diaphragm attached to an outer peripheral surface of a rear bearing case for supporting a spindle and an inner wall of a spindle head. The diaphragm supports the rear bearing case on an inner wall of a housing of the spindle head. As disclosed, the diaphragm supports the spindle while being elastically deformed in an axial direction but supports the spindle with high rigidity in a radial direction.